False Appearance, Movement, Surprise, and the Unseen Attacker bonus

Question

Related to How should I implement False Appearance? but that does not answer my questions about surprise and movement...

A Darkmantle has the "False Appearance" property which allows it to be indistinguishable from an ordinary stalactite while it remains motionless.

Suppose there is a Darkmantle on the ceiling of a cavern, 30 feet high. A party of adventurers walks in and cannot perceive the Darkmantle. It decides to attack. Since none of the party members can notice the threat, I believe they are all surprised at the start of the encounter, and unable to act during the first round.

PHB p.189:

Any character or monster that doesn't notice a threat is surprised at the start of the encounter. If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends.

But the party members are not in reach. On its turn, the Darkmantle needs to fly down and then make its attack action. Once it begins flying, it the party will most likely notice it. How the movement interacts with Surprise and Unseen Attacker is the part I'm most uncertain about.

1. Does the movement at the beginning of its turn ruin the surprise? (I think not)
2. Does the movement at the beginning of its turn ruin its opportunity to get advantage on the attack roll as an Unseen Attacker? (I think maybe, depending on Stealth: see #3)
3. Can the Darkmantle attempt to fly stealthily (Stealth check contested by each player's passive Perception) in order to maintain its Unseen Attacker advantage? (I think so)

How does this all play out for the DM? Suppose one member of the party's passive perception beats the Darkmantle's stealth as it flies down. Then do I (as DM) say, "Suddenly you, Aldore, notice one of the stalactites swooping down to attack. Everyone roll initiative, but because you didn't notice it before the start of its turn, none of you can take actions this round." And if its attacking anyone other than Aldore, does it get Unseen Attacker advantage on the attack roll, or is Aldore's shouted "look out!" negate that advantage? (Or can Aldore technically not shout a warning since he can't act? Often we do allow PCs to talk out of turn, but in a case of surprise, we'd go by RAW.)

Answer 1

The combat begins when the Darkmantle decides to attack. If the players don't notice the Darkmantle at the beginning of combat, they are surprised:

Any character or monster that doesn’t notice a threat is surprised.

So, thanks to False Appearance, the Darkmantle has a Surprise attack. Also, when the combat starts, the adventurers don't know that they're in a combat situation and so are not scanning the entire space all the time, unless previously stated (they are not combat-aware), so the Darkmantle remains hidden until its turn. However, all characters that are higher in the initiative order than the Darkmantle are no longer surprised at the end of their turn (or, better say, the "Surprise" property is no longer applied to them)! They are thus able to take reactions. For reference, this answer to another question (also linked in the comment of this question) breaks it down in full detail.

The Unseen Attacker advantage, however, depends on the DM's ruling, as the RAW are not specific about this.

As far as Darkmantles go, they could approach their target unnoticed. They don't have to fly, they can just drop down. One could reasonably fall unnoticed as easily as dropping a curtain on top of someone. Even if someone would notice the creature as it was falling, they'd only get enough time to look up and see the horrid array of teeth approaching swiftly.

If you still believe your Darkmantles are unable to automatically Batman your PC's, then a Stealth check is appropriate, and should be made during the Darkmantle's Move action. In either case, the Surprise Attack persists, it is only the Unseen Attacker advantage that is at stake.

Keep in mind that the target of the attack must be taken into account. If the Darkmantle attacks a character that was higher in the initiative order, the character can take reactions (like Tempest Cleric's Wrath of the Storm), regardless of whether the Darkmantle remained hidden during its movement or not. The very act of attacking reveals it:

If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses.

However, if the trigger for their reaction is before the attack, the Darkmantle's visibility has to be taken into account.

If the Darkmantle attacks a character that's lower in the initiative order, the character won't be able to react until the end of its own turn. So, even if his reactions trigger "on hit", he won't be able to use them. However, if the characters that went before the Darkmantle have a reaction that triggers "when an ally is attacked", they can take that reaction (again, because the attack reveals it).

There is only the final, special case to consider, where the Stealth check is only partially successful and one or more characters notice the Darkmantle as it moves in to strike. It is fully within their ability to warn the others of the threat, as they can talk out of turn (within reason). However, as it's the Darkmantle's movement that triggers the Stealth check, the players that were surprised at the moment of the attack still remain surprised. Whether the darkmantle retains its Unseen Attacker advantage or not is up to the DM and his opinion on how fast a character can react to a warning.

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From a role-playing perspective, if the Darkmantle remains unseen the adventurers either didn't notice the attack until the moment it hit, or they noticed it too late to do anything about it (including removing the advantage).

Here's an example of how that could be acted out:

(A PC walks under the Darkmantle and it decides to attack.)

DM: As you explore the dark cave, you feel a slight chill up your spine... Roll Initiative!

Player: Why? Do we see anyone here?

DM: Not yet.

(Initiative order is set. )

DM (to the players at the top of the initiative order, before the Darkmantle): You are not aware of any threats and proceed as you were.

(The Darkmantle's turn comes. It makes its optional Stealth check... and remains hidden. It swoops down on its unsuspecting target and rolls its attack with advantage... success!)

DM: Alright... Aldore, your head is suddenly wrapped in a curtain of flesh and you feel piercing teeth gnawing at your cranium. You take XX damage!

(DM now describes the attacker, and the battle continues. The characters that haven't had their turns yet are still surprised. When all turns resolve, the combat continues as usual from the top of the initiative order.)

Now, if the creature missed the attack (either by lacking or despite the advantage) you could have it be because the character noticed it and dodged in the last second, or you could make the Darkmantle comically flop on the ground in the middle of the party. Your choice :)

Answer 2

Surprise is dealt with pretty comprehensively in https://rpg.stackexchange.com/questions/65461/how-does-surprise-work-in-dd-5e

I will apply this to a specific example of a single Drakmantle and the party of Alice and Bob.

Alice and Bob (unknowingly) enter the Darkmantle's lair. The Darkmantle is "indistinguishable" from a normal stalactite or stalagmite; so Alice and Bob cannot detect it (as per How should I implement False Appearance?).

If Alice and Bob were experienced adventurers they may take precautions against the presence of a Darkmantle by e.g hitting every stalactite or stalagmite with a Firebolt; but let's assume they don't.

The Darkmantle chooses to initiate combat; at this point the rules on p. 189 of the Player's Handbook (PHB) kick in. Step 1 is "Determine Surprise. The DM determines whether anyone involved in the combat encounter is surprised." It is perfectly reasonable that the DM determines that Alice and Bob are surprised and the Darkmantle isn't.

It is also perfectly reasonable that the DM determines that Alice and Bob might be surprised:

The DM determines who might be surprised. ... Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side.

Both approaches have RAW support: in the first the DM decides that the Darkmantle is such a good ambush predator that surprise is automatic, in the second the Darkmantle rolls a Dexterity (Stealth) check with its +3 bonus and (probably) advantage: if that is better than either PCs passive Wisdom (Perception) score then that PC is surprised. In either case, the Darkmantle starts the combat both unseen and hidden.

Let's assume that both Alice and Bob are surprised (otherwise this gets boring). Step 2 is "Establish positions. The DM decides where all the characters and monsters are located." As stated in the question the Darkmantle is more than 5 feet from each PC and less than 30 feet from both

Step 3 is "Roll initiative. Everyone involved in the combat encounter rolls initiative, determining the order of combatants’ turns." Everybody rolls initiative and we will assume that the results give the order Alice, Darkmantle then Bob.

Step 4 is "Take turns. Each participant in the battle takes a turn in initiative order."

Alice is surprised so she cannot move or take an action on her first turn. Her turn therefore passes uneventfully and she is now no longer surprised!

Darkmantle can now move and attack (not necessarily in that order). At the start of its turn it is unseen and hidden. If it moves, this happens:

In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you. However, under certain circumstances, the Dungeon Master might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack before you are seen.

As DM you can rule that Alice and/or Bob are sufficiently distracted that the Darkmantle remains hidden or that they see it automatically: you are right either way. You can even split the difference and rule that Alice sees it because she is not surprised anymore but Bob doesn't because he is.

If it remains hidden to Alice it can move past her without provoking an opportunity attack. Notwithstanding, it can move past Bob without an opportunity attack because he can't take reactions. Anyone it is hidden from can be attacked with advantage: attacking means it is no longer hidden to anyone.

Bob (still surprised) can do nothing but is no longer surprised.

Round 2 begins with Alice's turn, the Darkmantle may still be hidden and unseen depending on what it did in Round 1 but surprise is no longer an issue.